Secondary batteries, being the most representative of the electrochemical devices, are devices which convert external electrical energy to chemical energy, store the electrical energy and generate electricity from the chemical energy when necessary. Secondary batteries, or “rechargeable batteries”, are designed to be recharged and used multiple times. Lead-acid batteries, nickel cadmium (NiCd) batteries, nickel hydrogen (NiMH) batteries, lithium ion batteries and lithium ion polymer batteries are frequently used as secondary batteries. Secondary batteries have lower costs of use and environmental impact than disposable primary batteries.
Secondary batteries are currently used in places where low power is needed, for example, devices for assisting the start up of car engines, portable devices, instruments and uninterrupted power supply systems. The recent developments in wireless communication technologies have led to the popularization of portable devices and have brought about a tendency for devices to connect to wireless networks. As a result, the demand for secondary batteries is growing explosively. In addition, hybrid vehicles and electric vehicles have been put into practical use to prevent environmental pollution, and by using secondary batteries in these next-generation vehicles, they reduce the weight and cost and extend battery life for long-term use.
Generally, most secondary batteries are cylindrical, prismatic or pouch type in shape because of their fabrication process. That is, a secondary battery is typically fabricated by inserting an electrode assembly composed of an anode, a cathode and a separator into a cylindrical or prismatic metal can or a pouch type case made of an aluminum laminate sheet, and injecting an electrolyte into the electrode assembly. Accordingly, the cylindrical, prismatic or pouch type secondary battery essentially requires a certain space for assembly, which is an obstacle to the development of various types of portable devices. Thus, there is a need for a novel type of secondary battery whose shape is easy to change, and particularly, an electrolyte that has high ionic conductivity without any risk of leakage.
Ionically conductive organic electrolytes predominantly used for conventional electrochemical devices based on electrochemical reactions are in the form of liquids in which salts are dissolved in non-aqueous organic solvents. However, the use of such electrolytes in the form of liquids causes degradation of electrode materials, increases the possibility of evaporation of organic solvents, and poses safety problems, such as fire and explosion resulting from high surrounding temperatures and increased battery temperatures. A risk of leakage and difficulty in realizing various types of electrochemical devices are additional safety problems. In attempts to overcome the safety problems of such liquid electrolytes, polymer electrolytes, such as gel polymer electrolytes and solid polymer electrolytes have been proposed. It is generally known that the safety of electrochemical devices increases in the order of liquid electrolytes, gel polymer electrolytes and solid polymer electrolytes, but the performance thereof decreases in the same order. It is known that electrochemical devices employing solid polymer electrolytes are not yet commercialized due to these inferior performances. Gel polymer electrolytes have low ionic conductivity, suffer from the risk of leakage and possess poor mechanical properties compared to liquid electrolytes.
Korean Unexamined Patent Publication No. 2008-33421 discloses an electrolyte using a plastic crystal matrix instead of using a liquid organic solvent. The electrolyte exhibits ionic conductivity comparable to that of a liquid electrolyte. However, the electrolyte exhibits very poor mechanical properties due to its flowability similar to that of liquid. In actuality, a separator is required to prevent short circuits in a battery using the electrolyte. In some cases, the introduction of linear polymer matrices, such as polyethylene oxide, is considered to improve the mechanical strength of plastic crystal matrix electrolytes. However, even in these cases, the electrolytes do not possess mechanical properties sufficient enough to replace the use of separators and because solvents are used, there exists a problem of having to add an additional drying process.
Thus, there is an urgent need to develop a solid electrolyte using a plastic crystal matrix electrolyte that has improved mechanical properties while maintaining high ionic conductivity of the plastic crystal matrix electrolyte.